Doctrine fetch join fails to hydrate correctly - doctrine-orm

Question

Original question
I am doing a fetch join in Doctrine, using a join table that has a composite key and no other defined fields, and getting incorrect data loaded into my entity. Is this a bug in Doctrine, or am I doing something wrong?
What follows is a simple example that illustrates the problem.
Create three entities:
/**
* #ORM\Entity
* #ORM\Table(name="driver")
*/
class Driver
{
/**
* #ORM\Id
* #ORM\Column(type="integer")
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #ORM\Column(type="string", length=255);
*/
private $name;
/**
* #ORM\OneToMany(targetEntity="DriverRide", mappedBy="driver")
*/
private $driverRides;
function getId() { return $this->id; }
function getName() { return $this->name; }
function getDriverRides() { return $this->driverRides; }
}
/**
* #ORM\Entity
* #ORM\Table(name="driver_ride")
*/
class DriverRide
{
/**
* #ORM\Id
* #ORM\ManyToOne(targetEntity="Driver", inversedBy="driverRides")
* #ORM\JoinColumn(name="driver_id", referencedColumnName="id")
*/
private $driver;
/**
* #ORM\Id
* #ORM\ManyToOne(targetEntity="Car", inversedBy="carRides")
* #ORM\JoinColumn(name="car", referencedColumnName="brand")
*/
private $car;
function getDriver() { return $this->driver; }
function getCar() { return $this->car; }
}
/**
* #ORM\Entity
* #ORM\Table(name="car")
*/
class Car
{
/**
* #ORM\Id
* #ORM\Column(type="string", length=25)
* #ORM\GeneratedValue(strategy="NONE")
*/
private $brand;
/**
* #ORM\Column(type="string", length=255);
*/
private $model;
/**
* #ORM\OneToMany(targetEntity="DriverRide", mappedBy="car")
*/
private $carRides;
function getBrand() { return $this->brand; }
function getModel() { return $this->model; }
function getCarRides() { return $this->carRides; }
}
Populate the corresponding database tables with this data:
INSERT INTO driver (id, name) VALUES (1, 'John Doe');
INSERT INTO car (brand, model) VALUES ('BMW', '7 Series');
INSERT INTO car (brand, model) VALUES ('Crysler', '300');
INSERT INTO car (brand, model) VALUES ('Mercedes', 'C-Class');
INSERT INTO car (brand, model) VALUES ('Volvo', 'XC90');
INSERT INTO car (brand, model) VALUES ('Dodge', 'Dart');
INSERT INTO driver_ride (driver_id, car) VALUES (1, 'Crysler');
INSERT INTO driver_ride (driver_id, car) VALUES (1, 'Mercedes');
INSERT INTO driver_ride (driver_id, car) VALUES (1, 'Volvo');
INSERT INTO driver_ride (driver_id, car) VALUES (1, 'BMW');
INSERT INTO driver_ride (driver_id, car) VALUES (1, 'Dodge');
Use this code to hydrate a Driver entity and display its contents:
$qb = $em->createQueryBuilder();
$driver = $qb->select('d, dr, c')
->from('Driver', 'd')
->leftJoin('d.driverRides', 'dr')
->leftJoin('dr.car', 'c')
->where('d.id = 1')
->getQuery()->getSingleResult();
print '<p>' . $driver->getName() . ':';
foreach ($driver->getDriverRides() as $ride) {
print '<br>' . $ride->getCar()->getBrand() . ' ' . $ride->getCar()->getModel();
}
Expected output:
John Doe:
BMW 7 Series
Crysler 300
Dodge Dart
Mercedes C-Class
Volvo XC90
Actual output:
John Doe:
BMW 7 Series
Dodge Dart
Dodge Dart
Volvo XC90
Volvo XC90
There is a strange duplication going on here with the associated entities. Specifically, child entity #3 is duplicated as #2, #5 is duplicated as #4, etc., and child entity #2, #4, etc. are not loaded at all. This pattern is consistent and reproducible.
Is there something wrong with my code? Why is Doctrine failing to correctly map the data from the database tables to the entities?
Additional remarks
The same problem occurs if one queries for all rides (aka associations between drivers and cars) and performs a fetch-join. This is an important case, because the answer by #qaqar-haider does not work for this scenario.
Assume the following table data
INSERT INTO driver (id, name) VALUES (1, 'John Doe');
INSERT INTO driver (id, name) VALUES (2, 'Erika Mustermann');
INSERT INTO car (brand, model) VALUES ('BMW', '7 Series');
INSERT INTO car (brand, model) VALUES ('Crysler', '300');
INSERT INTO car (brand, model) VALUES ('Mercedes', 'C-Class');
INSERT INTO driver_ride (driver_id, car) VALUES (1, 'Crysler');
INSERT INTO driver_ride (driver_id, car) VALUES (1, 'Mercedes');
INSERT INTO driver_ride (driver_id, car) VALUES (1, 'BMW');
INSERT INTO driver_ride (driver_id, car) VALUES (2, 'BMW');
INSERT INTO driver_ride (driver_id, car) VALUES (2, 'Crysler');
Imagine the following query with fetch-joins
$qb = $em->createQueryBuilder();
$rides = $qb->select('dr, d, c')
->from('DriverRide', 'dr')
->leftJoin('dr.driver', 'd')
->leftJoin('dr.car', 'c')
->getQuery()->getResult();
foreach ($rides as $ride) {
print $ride->driver->getName() . ': ' . $ride->getCar()->getModel();
}
Expected output:
John Doe: Chrysler
John Doe: Mercedes
John Doe: BMW
Erika Mustermann: BMW
Erika Mustermann: Chrysler
Actual output:
John Doe: Chrysler
John Doe: Mercedes
John Doe: Mercedes
Erika Mustermann: BMW
Erika Mustermann: BMW
Once again, the number of results is correct, but the associations are mixed up.
This is a simplified test case of the original question and is not subject to any WHERE or GROUP BY clause.

Answer 1

This is a bug in Doctrine. Unfortunately, the relevant Doctrine source code is quite ugly, so I am not certain that my bug fix is the best one, but it does fix the problem. I am copying the explanation from my bug report here for completeness:
The main cause of this bug is located in the ObjectHydrator class. In the hydrateRowData() method, $resultPointers is used to store a reference to the most recently hydrated object of each entity type. When a child entity needs to be linked to its parent, this method looks in $resultPointers to find a reference to the parent and, if it finds one, links the child to that parent.
The problem is that $resultPointers is an instance/object (rather than local/method) variable that does not get reinitialized every time hydrateRowData() is called, and so it may retain a reference to an entity that was hydrated the previous time the method was called rather than the current time.
In this particular example, the Car entity is hydrated before the DriverRide entity each time hydrateRowData() is called. When the method looks for the parent of the Car entity, it finds nothing the first time (because DriverRide has not yet been processed at all) and, on every subsequent call, finds a reference to the DriverRide object that was hydrated the previous time the method was called (because DriverRide has not yet been processed for the current row, and $resultPointers still retains a reference to the result of processing for the previous row).
The bug disappears when additional fields are added to DriverRide only because doing so happens to cause the DriverRide entity to be processed before the Car entity in hydrateRowData(). The duplication of records happens because some other weird part of this method causes the child entity to be identified as not fetch joined (and so lazy loaded) every other time the method is called (not counting the first time), and so those times (the third, fifth, etc.) the link between child and parent happens to turn out correctly.
I believe that the fundamental problem is that $resultPointers is neither a local variable nor reinitialized each time hydrateRowData() is called. I cannot think of any scenario in which you would need a reference to an object that was hydrated with data from the previous row of data, so I would recommend simply reinitializing this variable at the beginning of this method. That should fix the bug.

Answer 2

This is a nice question. I've spent some time trying to find out the answer, but I managed to achieve the expected result only by modifying the entity mappings - why the initial problem arises I still cannot understand, but I will continue to investigate.
So, this is what I've done. Instead of manually defining a DriverRide entity, let Doctrine define it for you with many-to-many mapping, as described here.
The entities will look like:
/**
* #Entity
* #Table(name="cars")
*/
class Car
{
/**
* #Id
* #Column(type="string", length=25)
* #GeneratedValue(strategy="NONE")
*/
protected $brand;
/**
* #Column(type="string", length=255);
*/
protected $model;
public function getBrand() { return $this->brand; }
public function getModel() { return $this->model; }
public function setBrand($brand) { $this->brand = $brand; }
public function setModel($model) { $this->model = $model; }
}
/**
* #Entity
* #Table(name="drivers")
*/
class Driver
{
/**
* #Id
* #Column(type="integer")
* #GeneratedValue(strategy="AUTO")
*/
protected $id;
/**
* #Column(type="string", length=255);
*/
protected $name;
// This will create a third table `driver_rides`
/**
* #ManyToMany(targetEntity="Car")
* #JoinTable(
* name="driver_rides",
* joinColumns={#JoinColumn(name="driver_id", referencedColumnName="id")},
* inverseJoinColumns={#JoinColumn(name="car", referencedColumnName="brand")}
* )
*/
protected $cars;
public function __construct()
{
$this->cars = new ArrayCollection();
}
function getId() { return $this->id; }
function getName() { return $this->name; }
function getCars() { return $this->cars; }
public function setName($name) { $this->name = $name; }
public function setCars($cars) { $this->cars = $cars; }
public function addCar(\Car $car) { $this->cars[] = $car; return $this; }
public function removeCar(\Car $car) { $this->cars->removeElement($car); }
}
And the query to get what you want:
$qb = $entityManager->createQueryBuilder();
/** #var $driver Driver */
$driver = $qb->select('d, dr')
->from('Driver', 'd')
->leftJoin('d.cars', 'dr') // join with `driver_rides` table
->where('d.id = 1')
->getQuery()
->getSingleResult();
printf("%s:\n", $driver->getName());
foreach ($driver->getCars() as $car) {
printf("%s %s\n", $car->getBrand(), $car->getModel());
}

Answer 3

You have to group it by a brand, othervise it would give you desired results
Group By brand
qb = $em->createQueryBuilder();
$driver = $qb->select('d, dr, c')
->from('Driver', 'd')
->leftJoin('d.driverRides', 'dr')
->leftJoin('dr.car', 'c')
->where('d.id = 1')
->groupBy('c.brand')
->getQuery()->getSingleResult();